CN102650020A - High-silicon high-manganese type high-thermal stability hot work die steel and thermal treatment process thereof - Google Patents

Abstract

The invention relates to a high-silicon, high-manganese type high-thermal-stability hot-working die steel and a heat treatment process thereof, which belong to the technical field of alloy steel manufacturing process. The steel is characterized in that the ratio of high content of silicon and manganese in the chemical composition remains unchanged at 1:1, and the mass percentages of the main alloying elements are: C0.25-0.45%, Si0.8-2.0%, Mn0. 8～2.0%, Cr3.5～4.5%, Mo0.6～1.2%, V0.4～0.8%, P＜0.02%, S＜0.02%, Fe balance. The preparation process of the hot work die steel of the present invention is as follows: batching, smelting, pouring, and then electroslag remelting; high-temperature homogenization heat treatment, then multi-directional forging heat processing; then superfine heat treatment and isothermal annealing treatment; finally Quenching and tempering heat treatment, that is, heating it to 980-1100°C for austenitization, and after oil cooling or water mist cooling, tempering at 540°C-600°C for two to three times. The hot work die steel of the invention has high thermal stability, high toughness and good thermal fatigue performance.

Description

High-silicon and high-manganese type high-thermal-stability hot work die steel and its heat treatment process

technical field

The invention relates to a high-silicon, high-manganese, high-thermal-stability hot work die steel and its heat treatment process. The die steel makes full use of the alloying characteristics of silicon and manganese to ensure high silicon and manganese content in the steel, and the two elements With the same content, it can not only reduce the alloy cost (compared to H13 series steel), but also has higher thermal stability, good toughness and thermal fatigue performance than H13 steel, and belongs to the technical field of alloy steel manufacturing technology.

Background technique

The alloying elements that play a role in high-temperature thermal strength and thermal stability in hot work die steel are usually Cr, Mo, V and other elements, so some current research work is mainly on the adjustment of these alloying elements. The recent research of the research group found that the addition of a certain amount of manganese can increase the matrix strengthening effect of the steel and delay the transformation of the martensitic structure, and improve the temper softening resistance of the steel. The successful case is the DM series developed by the research group. Steel for hot forging dies with high thermal strength. In addition, silicon is an effective element to improve tempering resistance. Increasing the content of silicon in steel can slow down the decomposition of martensite in steel during tempering, delaying the transformation from ε-carbide to θ-carbide. Reduce the growth rate of carbides in steel during tempering and improve the stability of carbides. The successful case is the high-silicon and low-molybdenum hot work die steel SDH3 steel developed by the research group.

The hot working die steels widely used in my country include tungsten-based 3Cr2W8V, chromium-based H13, and 5CrNiMo and 5CrMnMo used in hot forging dies. Although the tungsten-based 3Cr2W8V has high tempering resistance and high thermal strength, its plasticity, toughness, thermal conductivity and thermal fatigue performance are poor; H13 steel is the most widely used hot work die steel, but its The high-temperature strength is not very high, and the general use temperature cannot exceed 540°C; the thermal strength of 5CrNiMo and 5CrMnMo is low, and it is easy to cause the collapse of the working part of the mold. The hot extrusion die steel currently used in my country is 4Cr5MoSiV1 in the national standard GB/T1299-2000. The chemical composition of this hot extrusion die steel adopts C 0.32-0.45wt%, Cr 4.75-5.50wt%, Mo 1.20-1.75wt%, V 0.80-1.20wt%, Si 0.80-1.2wt%, Mn 0.20-0.5wt%, P≤0.03wt%, S≤0.03wt%. Since the chemical composition of this hot extrusion die steel contains relatively high molybdenum, chromium and vanadium elements and a certain amount of carbon elements, it belongs to hypereutectoid steel, so the segregation of its material electroslag ingot is serious, and there are A large number of large liquefied carbides make the material insufficient in toughness and prone to early crack failure. Because this material contains a large amount of secondary hardening elements, its tempered secondary carbides are easy to grow, coarsen and undergo type transformation under service conditions, and the alloying elements in the tempered martensite are also easy to precipitate and reduce the hardness of the steel. The strength, thereby reducing the high temperature performance of steel. The performance indicators of this steel are: after quenching at 1030°C + tempering at 590-610°C, the Rockwell hardness value is 44-46HRC, and the impact toughness value ("V" notch) Ak is ≥8J. Performance indicators such as hardness value and impact toughness value in the state are the key technical indicators of steel for hot extrusion dies, and are the main technical parameter indicators to measure the quality of steel for hot extrusion dies. In addition, the resistance to temper softening and thermal fatigue resistance are important performance indicators of hot work die steel.

The metallurgical manufacturing process of the above-mentioned hot work die steel is a process of electric furnace melting, electroslag remelting, and then forging. In its manufacturing process, after the electric furnace smelting and electroslag remelting process is completed, a 500Kg-3000Kg electroslag ingot is obtained, which is forged by a radial forging machine. This manufacturing process has the following problems: 1) The size of the electroslag ingot is small, and the small ingot size reduces the yield and manufacturing capacity of the product; 2) The original structure of the electroslag ingot has a large number of large particles or massive liquefied carbonization 3) The grain size of the material after forging is coarse, resulting in low impact toughness of the material after quenching and tempering, and the product grade is low, which cannot meet the market demand for high toughness hot work die steel need.

Under the current sustainable and economical development mode at home and abroad, this invention breaks the traditional research direction of high molybdenum from an economic point of view, adopts economical silicon and manganese as the main alloying elements, and makes full use of silicon and manganese alloying elements Solid solution strengthening and carbide tempering stability, etc., and reduce the content of noble alloy elements such as chromium, molybdenum and vanadium as much as possible, so as to develop a silicon-manganese ratio of 1:1 with high thermal stability and good impact toughness and thermal fatigue performance of hot work die steel.

Contents of the invention

The purpose of the present invention is to provide a low-cost chromium-molybdenum-vanadium series hot work die steel and its heat treatment process.

The hot work tool steel of the present invention is characterized in that the ratio of the high content silicon element and manganese element in the chemical composition of the steel remains unchanged at 1:1, and the mass percentage of each main alloy element is:

C 0.25～0.45%, Si 0.8～2.0%,

Mn 0.8～2.0%, Cr 3.5～4.5%,

Mo 0.8～1.2%, V 0.4～0.8%,

P<0.02%, S ＜0.02%

Fe balance.

The heat treatment process of the above-mentioned low-cost chromium-molybdenum-vanadium series hot work tool steel has the following process steps:

A. Using induction melting or electric arc furnace melting: according to the chemical composition and weight percentage of low-cost chromium molybdenum vanadium series hot work tool steel: C 0.25～0.45%, Si 0.8～2.0%, Mn 0.8～2.0%, Cr 3.5～4.5%,Mo 0.8~1.2%, V 0.4~0.8%, P<0.02%, S<0.02%, Fe balance, after batching, put it into induction melting or electric arc furnace for melting.

b. Electroslag remelting: The steel ingots poured out of smelting are placed in the electroslag remelting device for electroslag remelting.

c. High-temperature homogenization heat treatment: heat the steel ingot after electroslag remelting to 1180-1280°C for high-temperature homogenization treatment, keep it warm for 5-10 hours, uniform structure, and eliminate alloy composition segregation and liquid precipitated carbides.

d. Forging hot processing: adjust the above steel ingot temperature to the temperature range of 1050-1150°C for multi-directional forging; forging ratio ≥ 4, final forging temperature ≥ 850°C.

e. Ultra-fine treatment: the ultra-fine temperature is 1050-1150°C, and the ultra-fine time is 5-10 hours; then oil cooling or water mist cooling to below 250°C, and then heat back to the furnace, tempering at a high temperature of 600-700°C , Tempering and heat preservation for 2 to 4 hours.

F. Isothermal annealing treatment: the first-stage isothermal annealing temperature is 830-850°C, and the annealing time is 5-10h; the second-stage isothermal annealing temperature is 730-750°C, and the annealing time is 5-10h;

G. Quenching and tempering heat treatment: heating to 980-1100°C for austenitization, cooling to below 250°C with oil cooling or water mist; then tempering at 540-600°C.

In the high-temperature homogenization treatment, the temperature of the steel ingot is raised in multiple stages to ensure that the temperature inside and outside the steel ingot is uniform, that is, isothermal at 800°C and 1100°C respectively; After the temperature is uniform, forging treatment is carried out.

For the quenching and tempering heat treatment, heat to 980-1100°C with oil cooling or water mist cooling to 250°C, and then temper immediately. The holding time is 2 to 4 hours.

The theoretical basis of its composition design of hot work die steel of the present invention is as follows:

Compared with the general-purpose H13 hot work die steel, this hot work die steel appropriately reduces the content of Cr, Mo and V, increases the content of Si and Mn at the same time, and ensures that the ratio of Si and Mn is 1:1. Recent studies have found that the addition of a certain amount of manganese can increase the matrix strengthening effect of steel and delay the transformation of martensitic structure, improving the temper softening resistance of steel. Although manganese is a weak carbide forming element and cannot form carbide strengthening, the addition of a certain amount of manganese can promote the decomposition of cementite and delay the precipitation and growth of carbides, which is beneficial to the thermal stability of steel . In addition, manganese can increase and stabilize the content of retained austenite in steel, which can improve the toughness and thermal fatigue resistance of steel. Silicon element is not a carbide forming element, but silicon element is an effective element to improve tempering resistance. Increasing the content of silicon element in steel can mainly slow down the decomposition of martensite in steel during tempering. The tempering process after the transformation from tensite to martensite effectively hinders the decomposition of martensite, which is mainly by inhibiting the growth of ε carbides and expanding the stable area of ε carbides, delaying the transformation of ε-carbides to θ- carbide transformation. Silicon delays the transformation of ε→θ, and can fully reduce the growth rate of cementite in steel during tempering. Silicon atoms precipitate from the θ phase and form a silicon atom-enriched region around the θ phase, inhibiting the growth of the θ phase. Growth and coarsening; in addition, silicon can effectively improve the temper softening resistance of steel. Due to the strong affinity between V and carbon, it is easy to form VC primary carbides during the smelting process. This carbide has a large particle size, which not only does not improve the performance of the steel, but also reduces the toughness and thermal fatigue performance of the steel. It is difficult to completely eliminate it in the subsequent heat treatment process. Therefore, appropriately reducing the V content in steel can effectively reduce the proportion of VC primary carbides and improve the performance of steel. However, during the tempering process, V can reduce the decomposition rate of martensite and delay the transformation of austenite, and V forms MC-type secondary carbides, which are fine and dispersed, and are not easy to aggregate and grow. During the tempering process, Enhanced secondary hardening effect, greatly improving the thermal stability and impact toughness of steel. Therefore, the content of V in the steel is controlled between 0.4% and 0.8%, so as to give full play to the alloying effect of V. Cr mainly forms Cr23C6 carbides in hot work die steel, which are easy to precipitate along the grain and grow and coarsen, reducing the thermal stability and thermal fatigue performance of the material. The chromium content of this hot work die steel is somewhat reduce this adverse effect.

The thermal stability, impact toughness and thermal fatigue performance of the hot working die steel of the present invention are better than those of the H13 steel after the above heat treatment.

Description of drawings

Fig. 1 is the phase transformation point of the hot work die steel of the present invention.

Fig. 2 shows the annealed structure, quenched structure and tempered structure of the hot work die steel of the present invention after the above heat treatment process.

Fig. 3 is the tempering characteristic curve of the hot work die steel of the present invention under quenching at 1000-1090°C.

Figure 4 is a comparison of the thermal stability data of the hot work die steel of the present invention at 620°C with that of H13 steel.

Fig. 5 is a surface topography diagram of thermal fatigue cracks of the hot work die steel and H13 steel of the present invention.

Fig. 6 is a cross-sectional morphology diagram of thermal fatigue cracks in the hot work die steel and H13 steel of the present invention.

Fig. 7 is a comparison of the hardness gradient of the thermal fatigue section of the hot work die steel of the present invention and the H13 steel.

Detailed ways

Specific embodiments of the present invention are now described below.

Example

In this example, the composition and weight percentage of the hot work die steel are as follows:

C 0.32%, Si 1.2%, Mn 1.2%, Cr 3.8%, Mo 1.0%,

V 0.46%, P 0.01%, S 0.01%, the balance of Fe.

In the present embodiment, the technical process and steps of hot working tool steel are as follows:

A Electric furnace smelting: smelting in an electric arc furnace according to the above-mentioned ratio of alloying elements, the melting temperature is greater than 1500°C, casting into φ400mm-φ450mm electrode rods and cooling in air;

B Electroslag remelting: Place the poured steel ingot as a consumable electrode in the electroslag remelting device for electroslag remelting. The slag melting voltage is 56-62V, the current is 3000-5000A, and the electric system voltage is 57-59V. , current 11000-12000A, capping voltage 57-59V, current time 35-50Min, electroslag remelting into 500Kg-3000Kg electroslag ingot;

C High-temperature homogenization: heat the steel ingot after electroslag remelting to 1250°C for high-temperature homogenization treatment, keep it warm for 10 hours, uniform structure, and eliminate composition segregation and liquefied carbide;

D Forging: adjust the temperature of the above-mentioned high-temperature homogenized ingot to within the temperature range of 1100°C for multi-directional forging; forging ratio ≥ 4, final forging temperature ≥ 850°C;

E Ultra-fine treatment: the ultra-fine temperature is 1100°C, and the ultra-fine time is 8 hours; then oil cooling or water mist cooling to below 250°C, and then heat back to the furnace, tempering at a high temperature of 600-700°C, tempering Keep warm for 2-4 hours;

F Isothermal annealing treatment: the first-stage isothermal annealing temperature is 830-850°C, and the annealing time is 8h; the second-stage isothermal annealing temperature is 730-750°C, and the annealing time is 8h; then cool to room temperature with the furnace;

G Quenching and tempering treatment: the quenching temperature is 1030°C, oil quenching is used, and tempering is carried out twice at 560°C, each tempering is 2 hours.

After the hot work die steel of the present invention has undergone the above-mentioned smelting, thermal processing and heat treatment, the final product specification is Ø200mm round steel, and samples are taken for performance testing:

A phase transition point:

Ac1, Ac3, and Ms point test results are shown in Figure 1.

B Tempering characteristics:

The characteristic curves of tempering hardness versus tempering temperature after quenching at 1000°C, 1030°C, 1060°C and 1090°C respectively are shown in Figure 3.

C Hardness Testing:

Quenching hardness: 56.2HRC; Tempering hardness: 50HRC

D Impact toughness test:

Take a transverse impact sample on the billet, and the sample size is 7mm×10mm×55mm (according to the standard of North American Die Casting Association).

Impact energy value at room temperature: ≥280J.

E. Thermal stability:

The hot work die steel of the present invention is subjected to a stability comparison experiment with H13 steel at 620°C. After quenching and tempering, the H13 steel has the same hardness value as the steel of the present invention, both of which are 50HRC. The test results are shown in Figure 4 . It can be seen from accompanying drawing 4 that although the hardness value of the hot work die steel of the present invention is consistent with that of H13 steel before the start of the experiment, at 620°C, the hot work die steel of the present invention has Steel is better than H13 steel.

f Thermal fatigue performance test:

Under the condition of room temperature - 700 ℃, carry out cold and hot cycle, after 3000 times of cold and hot cycle, compare the thermal fatigue surface morphology and cross-sectional morphology and cross-sectional hardness gradient of the hot work die steel of the present invention and H13 steel (as shown in Figure 5- shown in Figure 7). It can be seen from the figure that after the thermal fatigue test of the hot work die steel of the present invention, the surface cracks are very uniform and small, and no relatively large main cracks are formed on the surface. However, the surface cracks of H13 steel are in the form of a network, and there are several main cracks with large widths, and the cracks are connected to each other in a cracked shape. In addition, it can be seen from the cross-sectional hardness gradient distribution that the hardness decrease of the H13 steel is significantly lower than that of the hot work die steel of the present invention. It can be seen from the comparison between the two that the thermal fatigue performance of the hot work die steel of the present invention is stronger than that of the H13 steel.

Claims (3)

1. the high manganese type of high silicon high thermal stability hot-work die steel, it is constant to it is characterized in that in the chemical ingredients of this steel that the ratio of element silicon and manganese element remains 1:1, and the mass percent of each main alloy element is: C 0.25～0.45%; Si 0.8～2.0%, and Mn 0.8～2.0%, and Cr 3.5～4.5%; Mo 0.6～1.2%, and V 0.4～0.8%, P＜0.02%; S＜0.02%, the Fe surplus.

2. the high manganese type of high silicon according to claim 1 high thermal stability hot-work die steel, it is constant to it is characterized in that in the chemical ingredients of this steel that the ratio of element silicon and manganese element remains 1:1, and the best in quality per-cent of each main alloy element is: C 0.32%; Si 1.2%, and Mn 1.2%, and Cr 3.8%; Mo 1.0%, and V 0.5%, P＜0.02%; S＜0.02%, the Fe surplus.

3. the thermal treatment process of the high manganese type of high silicon according to claim 1 and 2 high thermal stability hot-work die steel is characterized in that this technology has following steps:

A. smelt: chemical ingredients and weight percent by the high manganese type of high silicon high thermal stability hot-work die steel are: C 0.25～0.45%, and Si 0.8～2.0%, and Mn 0.8～2.0%; Cr 3.5～4.5%, and Mo 0.6～1.2%, and V 0.4～0.8%; P＜0.02%; S＜0.02%, the Fe surplus is prepared burden, induction melting or arc melting, is carried out esr then;

B. high temperature homogenization thermal treatment: homogenization temperature is 1180～1280 ℃, and the homogeneity time is 5～10h;

C. forge hot-work: will pass through high temperature homogenization thermal treatment steel ingot and be cooled to and carry out multiway forging processing in 1050～1150 ℃ of TRs, forging ratio >=4, final forging temperature >=850 ℃;

D. ultrafining heat-treatment: the super-refinement temperature is 1050～1150 ℃, and the super-refinement time is 5～10h; Cold soon then (oil cooling or water smoke are cold etc.) to below 250 ℃; Heat is sent stove back to again, at 600～700 ℃ of high temperings, and tempering insulation 2～4 hours;

E. isothermal annealing is handled: fs isothermal annealing temperature is 830～850 ℃, and annealing time is 5～10h; Subordinate phase isothermal annealing temperature is 730～750 ℃, and annealing time is 5～10h;

F. quenching and tempering thermal treatment: be heated to 980～1100 ℃, adopt oil cooling or water smoke to be cooled to below 250 ℃; Carry out 540～600 ℃ of temper subsequently, tempering 2～inferior, each tempering insulation 2～4 hours.

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